During early limb development several signaling centers coordinate limb bud outgrowth as well as patterning. Members of the T-box gene family of transcriptional regulators are crucial players in these processes by activating and interpreting these signaling pathways. Here, we show that Tbx15, a member of this gene family, is expressed during limb development, first in the mesenchyme of the early limb bud, then during early endochondral bone development in prehypertrophic chondrocytes of cartilaginous templates. Expression is also found in mesenchymal precursor cells and prehypertrophic chondrocytes, respectively, during development of skeletal elements of the vertebral column and the head. Analysis of Tbx15 null mutant mice indicates a role of Tbx15 in the development of skeletal elements throughout the body. Mutants display a general reduction of bone size and changes of bone shape. In the forelimb skeleton, the scapula lacks the central region of the blade. Cartilaginous templates are already reduced in size and show a transient delay in ossification in mutant embryos. Mutants show a significantly reduced proliferation of prehypertrophic chondrocytes as well as of mesenchymal precursor cells. These data suggest that Tbx15 plays an important role in the development of the skeleton of the limb, vertebral column and head by controlling the number of mesenchymal precursor cells and chondrocytes.

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Members of the Wnt family of secreted glycoproteins act as short-range signaling molecules in vertebrate embryogenesis. Previous work has shown that Wnt-4 is required for kidney development. Mice lacking functional Wnt-4 fail to form pretubular cell aggregates. Wnt-4 acts as an autoinducer of the mesenchymal to epithelial transition underlying nephron development. We have identified a member of the gene family encoding secreted frizzled related proteins (sFRP), putative Wnt antagonists, that shows overlapping expression with Wnt-4 in aggregating mesenchyme and simple epithelial bodies during metanephric development. sFRP-2 expression is absent in metanephric mesenchyme of kidneys mutant for Wnt-4 and is coinduced with Wnt-4 in isolated metanephric mesenchyme by cells expressing Wnt-4. The cysteine-rich domain of sFRP-2 binds to Wnt-4 as shown by coimmunoprecipitation experiments. Hence, sFRP-2 is a target of the Wnt-4 signaling pathway in the metanephric kidney and may modulate Wnt-4 signaling. sFRP-2 expression is highly dynamic and specific during other aspects of embryogenesis. sFRP-2 is expressed in subpopulations of ependymal cells in spinal cord and brain, in the developing eye, in limb bud mesenchyme, in the heart, and strongly in skeletogenic condensations of facial bones, suggesting widespread interaction with other members of the Wnt gene family during embryogenesis.

Juvenile nephronophthisis, an autosomal recessive cystic kidney disease, is the primary genetic cause for chronic renal failure in children. The gene (NPHP 1) for nephronophthisis type 1 has recently been identified. Its gene product, nephrocystin, is a novel protein of unknown function, which contains a src-homology 3 domain. To study tissue expression and analyze amino acid sequence conservation of nephrocystin, the full-length murine Nphp 1 cDNA sequence was obtained and Northern and in situ hybridization analyses were performed for extensive expression studies. The results demonstrate widespread but relatively weak NPHP 1 expression in the human adult. In the adult mouse there is strong expression in testis. This expression occurs specifically in cell stages of the first meiotic division and thereafter. In situ hybridization to whole mouse embryos demonstrated widespread and uniform expression at all developmental stages. Amino acid sequence conservation studies in human, mouse, and Caenorhabditis elegans show that in nephrocystin the src-homology 3 domain is embedded in a novel context of other putative domains of protein-protein interaction, such as coiled-coil and E-rich domains. It is concluded that for multiple putative protein-protein interaction domains of nephrocystin, sequence conservation dates back at least to Caenorhabditis elegans. The previously described discrepancy between widespread tissue expression and the restriction of symptoms to the kidney has now been confirmed by an in-depth expression study.